1. Field of the Invention
This invention relates to an article for improving the efficiency of operation of carbide-tipped cutting bits utilized in rotary machines adapted for pulverizing concrete and asphalt roadways and similar surfaces. More particularly, the invention relates to articles for improving the cutting action of rotatable carbide bits, and for protecting the support blocks in which the bits are mounted. Additionally, the invention relates to methods and apparatus for fastening ring-shaped cutting bit shields to the cylindrical shanks of cutting bits.
2. Description of Background Art
When concrete or asphalt roadways, aircraft runways, and the like require replacement or removal, a machine frequently employed for this purpose is a rotary pulverizer. Typically, the pulverizer utilizes a large drum which has a diameter of between two and three feet, and a length of six to twelve feet. Welded to the circumferential surface of the drum are 100-300 or more uniformly spaced, forged steel holding blocks which hold cylindrical, carbide-tipped cutting bits. The cutting bits extend radially outward from the drum. A typical rotary pulverizer drum is rotated by a 400 horsepower motor at a linear speed of 1,500 surface feet per minute.
The cutting bits used in many rotary pulverizers are elongated, solid cylinders, approximately 31/2" long by 3/4" in diameter. The outer or top face of the cylinder typically has a conical-shaped carbide top brazed to it. Each cutting bit is held in a forged steel holding block welded to the circumference of the pulverizer drum. Approximately one-half of the length of the bit extends outward from the top face of the holding block. Each cutting bit is secured in its holding block with a clip or sleeve that permits the bit to rotate freely about its cylindrical axis. This permits the bit to be rotated by tangential frictional contact with the material which it is used to cut. As a result of rotation of the bit, wear of the bit is more evenly distributed, extending the useful life of the bit. U.S. Pat. No. 4,201,421 discloses a split sleeve for rotatably mounting cutting bits in their holding blocks.
Although providing the capability for free rotation of the cutting bit results in more uniform wear and extended life of the bit, wear of the bit holding block continued to be a problem. The flat, upper face of the bit holding blocks is continuously impacted with abrasive materials during the operation of the rotary pulverizers. Also, if a bit wears down to the extent that it extends only a short distance out from the face of its holding block, more rapid and destructive wear of the holding block occurs. Excessive wear of the bit holding block requires that the worn blocks be removed from the pulverizer drum with a cutting torch, and a new block welded onto the drum. This is a time consuming and therefore costly operation. Furthermore, it frequently happens that replacement of holding blocks under field conditions results in a misalignment of the bore axis of the mounting block from its optimum orientation.
To alleviate the problem of cutting bit holding block wear, I introduced in October of 1983 an accessory which I referred to as the "Spin Shield." This accessory is comprised essentially of a flat, hardened steel annular ring or washer which is adapted to fitting between the enlarged base of a cylindrical cutting bit, and the transverse outer face of the holding block which rotatably supports the bit. In addition to absorbing wear which would otherwise be experienced by the bit holding block, my "Spin Shield" provided other advantages. One such advantage is the separation between the cutting bit chamfer from the bit holding block, preventing the formation of a burr on the inside of the bore of the holding block. Another advantage is the reduction of cutting bit friction, allowing the cutting bit to rotate more freely. This results in cooler operation and more even wear of the cutting bit, substantially extending its life.
Subsequent to my introduction of the "Spin Shield" bit holding block protector, I observed in the field a cutting bit with an integral flange near the middle of the forged bit shank. The apparent purpose of the flange was to achieve in a limited way some of the advantages of my "Spin Shield."
In my U.S. patent application Ser. No. 06/677,235 filed Dec. 4, 1984, I disclosed a "Pulverizer Cutting Bit Shield" to protect holding blocks for cylindrical cutting bits from excessive wear. The cutting bit shield disclosed in that application comprised an annular steel ring shaped like an inverted dish with a central hole and a substantially curved, convex upper surface. The ring is adapted to being attached coaxially over the cylindrical shank of a cylindrical cutting bit by inserting the upper portion of the cutting bit into the bottom entrance of the central hole. The upper surface of the ring is then pounded down on the shank until it abuts an upper annular flange on the shank of the bit, and is retained in position by a tight interference fit between the outer diameter of the shank and the inner diameter of the ring.
The pulverizer cutting bit shield disclosed in my above-referenced patent application provides an effective means for protecting cutting bit holding blocks from excessive wear. Moreover, the rotation of the pulverizer cutting bit in unison with the shield fixed to its shank is particularly effective in applications where it is desired to mix material such as soil impacted by the cutting bit.
However, I have found that for some applications of pulverizer cutting bits, it would be desirable to have a freely rotatable cutting bit shield. In particular, for those applications of pulverizer cutting bits in which high impacts and/or highly abrasive materials are encountered, a freely rotating bit shield would be better because normal wear is distributed evenly on the cutting bit, thereby extending the useful life of the bit.
The present invention is responsive to the need for an accessory for cylindrical pulverizer cutting bits which affords protection for the holding block in which the bit is mounted, and which is also freely rotatable with respect to the shank of the cutting bit.